1. Field of the Invention
The invention relates to a method of electrolytic metal coating of a strip-shape metal substrate and to apparatus for carrying out the method.
2. Description of the Prior Art
EP-A No. 268823 describes electrolytic coating of metal strip in which the substrate is conveyed in a continuous process as cathode through an electrolytic coating apparatus of a type with an insoluble anode. Cathode and anode are connected to an external voltage source, so that metal ions are deposited onto the substrate from an acidic electrolyte between cathode and anode to form the desired plated coating.
In the following description the invention will be described mainly as applied to electrotinning of steel. However, the invention may equally be used for coating a metal substrate other than steel and for coating a metal substrate with a metal coating other than tin.
In the classical process of tinplating of steel, a soluble anode is used of the same metal as the coating to be formed, that is to say tin. This tin anode, is consumed during the coating process because the tin anode goes into solution in the electrolyte according to the electrochemical reaction: EQU Sn.fwdarw.Sn.sup.2+ +2e.sup.- ( 1)
A coating of tin is deposited from the electrolyte onto the cathode, that is to say the cold rolled steel strip, according to the electrochemical reaction: EQU Sn.sup.2+ +2e.sup.- .fwdarw.Sn (2)
This classical tinplating method has various disadvantages of which the most important mentioned here is that, during the process, because the tin anode goes into solution, the distance between anode and cathode varies and as a result the tin coating thickness on the cathode is uneven.
In order to prevent these disadvantages, it is known, from for example EP-A No. 268823, to use an insoluble anode instead of the tin anode. A coating of tin is deposited onto the cathode from the electrolyte according to the same electrochemical reaction (2) as in the classical process. The following electrochemical reaction takes place on the anode: EQU 2H.sub.2 O .fwdarw.4H.sup.+ +4e.sup.- +O.sub.2 ( 3)
The oxygen formed escapes. Water is consumed overall, the H.sup.+ -concentration in the electrolyte increases and the Sn.sup.2+ -concentration decreases. In a continuous process these changes are compensated for in a separate tin dissolving apparatus.
A disadvantage of this known process with an insoluble anode and an acid electrolyte is that the technical service life of the anode, which may consist of for example iridium oxide (IrO.sub.2) on titanium or of platinum on titanium, is very limited.
U.S. Pat. No. 3669852 described removal of dissolved oxygen during electroplating of gold from citrate and phosphate buffered gold cyanide plating baths by means of nitrogen gas or a chemical reducing agent such as hydrazine or sulphite ions. Chemical Abstracts vol. 104, 1986, no. 22301 mentions use of H to decrease energy consumption in electrowinning and in fuel cells, by anode depolarization. See below for a further discussion of use of hydrogen in fuel cells.